1. Field of the Invention
The present invention relates to a digital modulating/demodulating apparatus for transmitting or recording a digital signal and a digital demodulating apparatus.
2. Description of the Related Art
In a conventional digital modulating/demodulating apparatus, 8 phase shift keying (8PSK) is generally used. According to 8PSK, a code of 3 bits (8-ary code) is mapped to one of eight code points on a single circle arranged on a constellation plane. The eight code points are arranged on the single circle with the same interval. The single circle has a center positioned on the origin of the constellation plane.
According to 8PSK, a code point corresponding to a current 8-ary code C(n) is determined in accordance with a code point corresponding to a previous 8-ary code C(n-1) and an angle defined by a value of the current 8-ary code C(n). Such an encoding method in which multiple-valued information is represented by a phase difference between the code point corresponding to the previous 8-ary code C(n-1) and the code point corresponding to the current 8-ary code C(n) is referred to as "a differentially encode". The differentially encode is described in detail in John G. Proakis, "Digital Communications", pp.265-272, McGraw-Hill Book Company, New York. An I-signal and a Q-signal indicating a code point are quadrature modulated into a modulated signal. The modulated signal is transmitted or recorded. Then, the modulated signal is demodulated into a demodulated I-signal and a demodulated Q-signal. The demodulated I-signal and the demodulated Q-signal are decoded into a digital signal by executing the inverse of the process described above.
However, in the conventional modulating/demodulating apparatus in which eight code points are arranged on a single circle with the same interval on a constellation plane, a phase shift of a reproduced carrier is likely to occur due to a Jitter (fluctuation in time-axis) which is caused in the transmission path or the recording/reproducing unit. The phase shift of the reproduced carrier causes a signal point defined by a demodulated I-signal and a demodulated Q-signal shifted in a phase direction. Since adjacent code points on the single circle are only 45 degrees away from each other in the phase direction, an error rate of decoded data is greatly affected by an amount of phase shift of the signal point. As a result, the error rate of the decoded data varies drastically depending upon the amount of phase shift of the signal point.